[BUGFIX release] Tests for triggerEvent's many argument signatures

triggerEvent has many arities (some were untested):

```
selector, type
selector, type, options
selector, context, type
selector, context, type, options
```

see https://github.com/emberjs/ember.js/issues/5265 and https://github.com/emberjs/ember.js/pull/5165#discussion_r14915117
https://github.com/emberjs/ember.js/pull/5281